1. Field of the Invention
This invention relates to an optical switch for switching the transmission path of an optical signal in accordance with a change in refractive index due to carrier injection, and particularly to an optical switch that enables reduction in injected current by having such a structure that injected effective carriers tend to be accumulated in an part where they will be effective, and constricting the current injection region to improve the efficiency.
2. Description of the Related Art
The present communication networks such as LAN and WAN (Wide Area Network) usually employ a communication system of transmitting information through electric signals.
A communication method of transmitting information through optical signals is only used in trunk networks for transmitting a large quantity of data and some other networks. Further, these networks use point-to-point communication and have not reached a level of communication network that can be called “photonic network”.
To realize such a photonic network, devices such as an optical router and an optical switching hub are necessary that have the same functions as a router, a switching hub and the like for switching destinations of electric signals.
These devices need an optical switch for switching the transmission path at a high speed. There are optical switches using ferroelectrics such as lithium niobate and PLZT (lead lanthanum zirconate titanate), or an optical switch in which a waveguide is formed on a semiconductor and carriers are injected into the semiconductor to change the refractive index and thus switch the transmission path of an optical signal.
The following are literature of prior arts related to the conventional optical switch in which a waveguide is formed on a semiconductor and carriers are injected into the semiconductor to change the refractive index and thus switch the transmission path of an optical signal.
Patent Reference 1: JP-A-04-320219
Patent Reference 2: JP-A-05-249508
Patent Reference 3: JP-A-06-059294
Patent Reference 4: JP-A-06-062450
Patent Reference 5: JP-A-06-130236
Patent Reference 6: JP-A-06-289339
Patent Reference 7: JP-A-08-082810
Non-Patent Reference 1: “2×2 Optical Waveguide Switch with Bow-Tie Electrode Based on Carrier-Injection Total Internal Reflection in SiGe Alloy,” Baojun Li and Soo-Jin Chua, p.206-p.208, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.13, NO.3, MARCH 2001
Non-Patent Reference 2: Baujun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Chengwen Pei and Xun Wang, Appl. Phys. Lett., pp.1-3, 75 (1999)
Non-Patent Reference 3: K. Ishida, H. Nakamura, H. Matsumura, T. Kadi, and H. Inoue, Appl. Phys. Lett., pp.141-142, 50(3), 19(1987)
FIG. 1 is a plan view showing an example of a conventional optical switch. FIG. 2 is a sectional view along a line A-A′ in FIG. 1.
In FIG. 1, a waveguide layer 2 having an X-shaped waveguide is formed on a substrate 1, and a rectangular N-electrode 3 is formed at the intersecting part of the X-shape.
Near the intersecting part of the X-shaped waveguide, a rectangular P-electrode 4 is formed parallel to the N electrode 3.
FIG. 2 is a sectional view along the line A-A′ in FIG. 1. In FIG. 2, a p-SiGe layer 2a having the X-shaped waveguide is formed on the p-Si substrate 1. Electrons are injected into the N-electrode 3, and holes are injected into the P-electrode 4. In this example, the carrier injection is carried out by causing a forward current to flow to an Si PN diode formed by an n+ layer formed below the N-electrode 3 and a p+ layer formed below the P-electrode 4.
The operation in the conventional example shown in FIG. 1 will now be described with reference to FIG. 1. When the optical switch is off, no current is supplied to the N electrode 3 and the P-electrode 4.
Since the refractive index at the intersecting part of the X-shaped waveguide 2 does not change, for example, an optical signal entering from the part “PI01” in FIG. 1 travels straight through the intersecting part and is emitted from the part “PO01” in FIG. 1.
On the other hand, when the optical switch is on, electrons are injected from the N electrode 3 and holes are injected from the P-electrode 4. As a result, the carriers (electrons and holes) are injected into the intersecting part.
Since the refractive index at the intersecting part of the X-shaped waveguide is changed to be lower because of a plasma effect, for example, an optical signal entering from the point “PI01” in FIG. 1 is totally reflected by the part of low refractive index generated in the intersecting part and is emitted from the part “PO02” in FIG. 1.
As a result, by supplying a current to the electrodes, thus injecting carriers (electrons and holes) into the intersecting part of the X-shaped waveguide and controlling the refractive index at the intersecting part, it is possible to control the position where an optical signal is emitted, that is, to switch the propagation path of the optical signal.
Meanwhile, the carrier-injected optical switch can make the operation easier by decreasing driving current and thus reducing the burden on the driving circuit.
In this example, carrier injection is carried out by causing a forward current to flow to an Si PN diode. In such a structure, injected carriers are not accumulated in an intermediate part and easily flow out to the opposite electrode. Therefore, a very large current must be supplied. This causes a large burden on the driving circuit and also makes high-speed operation difficult.
That is, as injected carriers are not accumulated in an intermediate part and easily flow out to the opposite electrode, a very large current must be supplied for the operation, causing problems of a large burden on the driving circuit and difficulty in high-speed operation.
FIGS. 3 and 4 show another conventional example of an optical switch. FIG. 3 is a plan view. FIG. 4 is a sectional view along a line A-A′ in FIG. 3.
In FIGS. 3 and 4, a first clad layer 20 formed by an n-InP layer, an X-shaped waveguide 21 comprising an n-InGaAsP layer, a second clad layer 22 comprising an n-InP layer and a contact layer 23 comprising an n-InGaAsP layer are stacked on a semiconductor substrate 1a made of InP.
An insulation layer 24 made of SiO2 or the like is formed over the entire surface except for the intersecting part of the X-shaped waveguide 2b (see FIG. 3), and a first electrode 25 is formed at the intersecting part.
In FIG. 4, at a part indicated by “A” in the second clad layer, right below the P-electrode 25, and parts indicated by “B” and “C” in the parts of the first clad layer except for the part right below the P-electrode, Zn, which is p-type impurity, is diffused to form a current narrowing structure.
That is, in the structure of this example, the p regions are provided in the waveguide 21 of the switch part to narrow the current, and the region of a high carrier concentration is regulated to limit the region where the refractive index changes. On the rear side of the InP substrate 1a, an N electrode 26 is formed.
The operation in the conventional example shown in FIGS. 3 and 4 will now be described. When the optical switch is off, no current is supplied to the first electrode 25 and the electrode on the rear side of the substrate 1a. 
Since the refractive index at the intersecting part of the X-shaped waveguide 2b does not change, for example, an optical signal entering from part Pi in FIG. 3 travels straight through the intersecting part and is emitted from part P1.
On the other hand, when the optical switch is on, a current is supplied from the first electrode 25 to the electrode 26 on the rear side of the substrate 1a, and carriers (electrons and holes) are injected into the intersecting part.
As a result, since the refractive index at the intersecting part of the X-shaped waveguide 2b directly below electrode 25 is changed to be lower because of a plasma effect, for example, an optical signal entering from the point “Pi” is totally reflected by the part of low refractive index generated in the intersecting part and is emitted from a part indicated by “P2”.
As a result, by supplying a current to the electrodes, carriers (electrons and holes) are injected into the intersecting part of the X-shaped waveguide 2b and thus the refractive index at the intersecting part is controlled, and it is possible to switch the propagation path of the optical signal.
However, in the conventional example shown in FIGS. 3 and 4, as the p regions for current narrowing are provided right below the InGaAsP layer, which is an optical confinement layer of the waveguide part, the light propagating through the waveguide spreads to the p regions having a high carrier concentration, and free carrier absorption occurs. This causes a problem of increase in propagation loss of the waveguide.